For example, what is the reason why greenhouse gases reflect heat from the Earth back to Earth, but not heat from the Sun back to the space?
From what I understand, it has something to do with radiation and emitted waves, but what exactly makes the difference?
In: Planetary Science
Greenhouse gases hold on to the heat better than “normal” gases. It’s like heating something up and then covering it with a blanket. A greenhouse gas is the blanket
The energy that travels from the sun to the Earth comes mostly in the form of UV and visible light. That radiation is absorbed by the ground which is then heated up. As it warms, it begins to emit infrared radiation which CO2 and other trace molecules tend to absorb. The end result is that the atmosphere warms up.
Heat radiation have different frequencies depending on the temperature. The ambient temperature on Earth means that the heat radiation is infrared. If you heat thing up they will start glowing red from their heat radiation. The Sun is so hot that it is glowing white and a lot of the heat radiation is even into the ultraviolet range.
The greenhouse effect happens when things are transparent to the higher frequencies, like yellow, blue and ultraviolet, but absorb or reflect lower frequencies like red and infrared. This means that the heat radiation will only go from the hot to the cold, not the other way around.
The simple answer is that the greenhouse effect is not actually a reflection of heat back at the Earth. It just gets reported like that because that’s a simple analogy people can easily understand.
[Here](https://www.youtube.com/watch?v=oqu5DjzOBF8) is a really good video explaining in much more detail.
Water, methane, CO2 and other GHGs (HFCs etc.) absorb infrared radiation and then re-emit it (you can tell this from looking at IR spectra of organic compounds).
The radiation from the Sun is a broad spectrum of wavelengths/frequencies so while it does contain infrared, there’s a lot more besides.
Earth absorbs a certain percentage of this radiation (based on the albedo of the location) and then re-emits it BUT it re-emits pretty much all of it as infra-red, which the gases can trap.
It is all down to the wavelengths. Light (visible light) comes from the Sun and passes through the atmosphere, it then hits the ground and warms the ground up. The warm ground then radiates infrared radiation (different part of the spectrum of light) and that doesn’t pass so easily through the atmosphere and basically bounces back down to the Earth off the Carbon Dioxide gas. and so the heat stays “trapped” and warms the planet up. https://youtu.be/_vFRSAs9DiY
Heat from the sun is almost exclusively the “radiated” type, meaning that photons carrying energy travel from the sun to us.
Those photons are absorbed by things which then heat those things up – if you stand in the sun during the day, you can feel the heat from the sun.
This heats up the whole planet.
The only way this heat can then escape the planet is if it is radiated away, because the other methods of getting rid of heat don’t work in a vacuum – conduction requires materials touching and convection requires fluid to move.
CO2 and other green house gasses absorb radiated heat (from the sun, as well as from the planet) and then heats up the atmosphere through conduction and convection.
If I have a bright LED it might not be that warm and light doesn’t heat the air much. But if shine it at a black wall, the temperature starts to rise. Because when something “absorbs light” it turns it into heat. The sun puts out a ton of visible light and even higher energy UV light that isn’t exactly hot on its own, until it hits something like the earth, at which point it turns into more heat.
Check out this video.
Light/ energy from the sun enters as visible light which passes right through our atmosphere. It then mostly turns into infrared as it is absorbed by the earth/oceans. This infrared is trapped as it cannot pass through the atmosphere (video shows glass blocking infrared the same as our atmosphere does.
Radiation that comes from the sun is mostly visible light. Visible light passes through the atmosphere mostly unimpeded.
If this visible light hits something like a mirror or a very reflective surface (snow, ice, etc.), it simply gets reflected into space, and the atmosphere also doesn’t impede that light on the way back up.
However, a lot of visible light is not reflected. It is absorbed by the surface, which warms it. Over time, any warm surface emits its heat back into space, thats why it’s getting colder during the night … only Earths emission does not happen in the form of visible light (that would require a surface that is thousands of degrees hot), but rather in infrared light. It is this infrared light that greenhouse gasses are so good at absorbing.
And thats why the atmosphere absorbs radiation energy mostly only in one direction.
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